The human eye sees a black-and-white border collie, about 35 pounds, stepping slowly and silently across the lawn at Fort McHenry, its light-brown eyebrows lending its face a most personable expression.

The Canada goose eye apparently sees something very different: a predator, perhaps a fox or wolf.

Standing at Whetstone Point about 50 feet from the dog Thursday morning, more than 40 geese sense trouble, honk, begin ambling away. As the dog steps closer, the birds take wing and head for the safety of the water.

Boo the border collie, energetic employee of Geese Police of Maryland, has done his job. For now, at least, the Fort McHenry National Monument and Historic Shrine in South Baltimore is goose-free.

He'll be back, though, and soon, as the yearly migration has just begun. He and his handler, Rich LaPorta, are in the first week of a yearlong $19,020 contract at Fort McHenry, as the historic site fights a continuing battle with Canada geese — both migratory and Maryland populations — or, more specifically, their droppings.

"We have, for years, dealt with their excrement," said Paul Bitzel, a resources manager for the National Park Service assigned to Fort McHenry and the Hampton National Historic Site in Towson. While visitor traffic is up for the War of 1812 bicentennial, with commemorative events to continue next year, Bitzel said "it's a problem for us whether we have a large visitation due to the bicentennial or not."

Bitzel said he appreciates the sight and sound of the geese as part of the scenery at the 42-acre grounds by the Patapsco River, but goose poop is a common complaint. Some visitors, not realizing it comes from birds, ask why the park is letting dogs run loose.

Experts say one adult goose can drop a pound or two of waste a day. Considering the numbers — 51,000 resident Canada geese estimated in Maryland last spring, and 462,000 migratory birds in last January's survey — the problem can mount considerably.

Bitzel said park workers tried using a goose repellent made from a grape juice byproduct, but at $139 a gallon, it was too expensive, and also labor-intensive: it had to be applied with a backpack sprayer repeatedly, especially after rain.

Letting the grass grow high could work, as geese prefer the low grass — the better to keep an eye out for Boo-like menaces — but that wouldn't fit the aesthetics of the park.

Years ago, through a park service colleague, Bitzel heard about border collies and their way with geese. They were being used successfully to shoo birds from waterfront parks in New York, the Statue of Liberty and Ellis Island.

In 2010, LaPorta said, the company had a three-month tryout, then returned the next year for about half the year and for a full year in 2012.

The company recently won the annual contract bid again, making the National Park Service one of about 25 clients in Central Maryland between Cecil and Prince George's counties, including golf courses, federal buildings, universities, cemeteries and the CNX Marine Terminals in South Baltimore.

LaPorta's company is a division of Geese Police, founded in 1996 by a former Connecticut golf course groundskeeper, now with franchises in nine states.

David Marcks, like Bitzel, tried several tactics to ward off geese: live swans, fake dead geese, fences around ponds, goose repellent.

Border collies, he found, proved the best way to drive off geese in a humane way without running afoul of the U.S. Migratory Bird Treaty Act, which protects the birds whether they migrate or not.

The dogs don't touch the geese — in many cases they never come within the distance of a friendly toss of a crust of bread. They want to herd the birds, not hurt them, as other breedsmight, even if the geese find them scary.

Something about their stalk, even the look in their eye, appears to convey danger. Boo, for instance, moves less like a dog than a cat — head low, padding silently toward the prey.

"When the dog's out there, they're looking at it like it's a predator out there," said LaPorta, who takes care of Boo and his colleagues Joe, Glen and Millie at his home in northern Baltimore County.

While LaPorta has to return to his clients for repeat visits, he said the geese dwindle as visits continue, suggesting the fear that the dogs instill in the birds lingers even after they have gone.

Why is hard to say, but it does appear to work, said Lynsey White Dasher, the director of humane wildlife conflict resolution for the Humane Society of the United States. She said she cannot point to any formal studies of the subject, but anecdotal accounts "have shown the effectiveness of the dogs."

And not just any dogs. Border collies seem to work better than just having another type of dog go chasing the birds and barking, she said. She's heard about the border collie's "crazy eye," meaning its predatory stare, and wonders if their feline or fox-like movements might explain it.

In any case, she said, the society approves of this approach as a humane way to chase geese, with conditions including dog training and limiting the pursuit chiefly to the fall and winter — when the birds are not nesting, guarding their newborn goslings, or molting and unable to fly.

LaPorta said he observes limits, and often has to explain to bystanders that this method of goose control is considered humane. Bystanders ask what he does when he catches the geese, and he has to explain that he doesn't catch them.

He answers all sorts of questions, he said, including: "'How do I get my kids to listen like your dog?'"

He and Boo had a busy day Thursday, herding geese at 11 stops in Baltimore and Anne Arundel counties, then circling back to Fort McHenry around closing time at 5 p.m.

"There wasn't a goose around," said LaPorta, who saw as many 90 geese at the fort in the morning early this week. "They're starting to get the hint already, and they're staying away."